Preparation Of Magnesium Doped Hydroxyapatite And Their Adsorption Of Heavy Metals Ion

Heavy metal wastewater pollution has become an increasingly prominent environmental problem.It is the important research direction that development of new efficient adsorbent materials for the treatment of heavy metal wastewater.Hydroxyapatite?HAP?has the characteristics of good biocompatibility,no secondary pollution and widely source,and can effectively adsorb heavy metals in wastewater.It is one of the hottest new environmental functional materials.In order to prepare an adsorbent material with good properties,magnesiumdoped hydroxyapatite was synthesized by the method of doping Mg²⁺ in this paper.The effects of different magnesium doping dose on the removal of Pb²⁺ and Cu²⁺ were studied.Magnesiumdoped hydroxyapatite were characterized by SEM,BET,FITR,XRD and XPS.Adsorption thermodynamics and dynamics were studied,and the adsorption mechanisms also were analyzed.The main results were as follows:?1?Using the sol-gel method to synthesize hydroxyapatite,the effects of calcined temperature,solution pH and aging temperature on adsorption of Cu²⁺,Pb²⁺.The optimum conditions of preparation HAP were that calcined temperature is 300 ?,the value of pH belong to 10,and aging temperature is equal to 70 ?.The calcined temperature is a key factor,which was affected the adsorption performance of synthetic HAP.When the calcined temperature was higher than 300 ?,the adsorption capacity of Pb²⁺ and Cu²⁺ decreased sharply,which was mainly caused by the decomposition of HAP.?2?A series of magnesium doping hydroxyapatite?MHAPs?were synthesized byMg²⁺ instead of Ca²⁺ in the preparation process,which were respectively named of 0.25 MHAP ? 0.5MHAP ? 0.75 MHAP ? MHAP.It was found that the doping of magnesium ions reduced the particle size,pore size and specific surface area of apatite,and specific surface areagradually increased with the increase of magnesium ion.When Mg / Ca substitution ratio is 100%,the specific surface area of MHAP increased from 22.77m2/g to 50.72m2/g compared with unmodified HAP,and the BET particle size dropped from 26.36 nm to 11.83 nm.After magnesium doping into MHAP,the number of surface active group,such as –OH was increased.?3?The adsorption behavior of Cu²⁺ and Pb²⁺on different magnesium doping dose hydroxyapatite were studied.The results showed that the best doping ratio of magnesium was 100%,where had the highest adsorption capacity of Cu²⁺ and Pb²⁺,and respectively the value of 337.98mg/g and 2180.3mg/g at 303 K.Compared with unmodified HAP,they were respectively increased by 213.64 mg / g and 1470 mg / g.The adsorption capacity of MHAP to Pb²⁺was stronger than Cu²⁺.When the initial concentrationwas less than 500mg/L,the adsorption efficiency of MHAP of Cu²⁺was above 94%,and the removal rate of Pb²⁺ was 100%.While the initial concentration of Pb²⁺ was 5500mg/L,the removal rate of MHAP was also higher than 80%.The effect of pH on adsorption is mainly dependent on the distribution of heavy metal ions.The adsorption capacity improvedwith the increasingof p H,which was mainly due to enhanced the hydrolysis of Cu²⁺ and Pb²⁺.The adsorption of Cu²⁺ and Pb²⁺ on MHAP can be spontaneously and endothermic.The increase of temperature is helpful to improve the adsorption capacity,which is in accordance with Langmuir isothermal and pseudo-secondary adsorption kinetics model.?4?The adsorption capacity of Pb²⁺,Cu²⁺ and Zn²⁺ at room temperature was 9.5mmol/g,5.82mmol/g,5.0mmol/g,The adsorption capacity was Pb²⁺> Cu²⁺> Zn²⁺.At low concentration,the competition behavior of three kinds of ions was weak,and the competition enhanced with the increase of initial concentration.When three ions coexisted,the adsorption capacity ofPb²⁺,Cu²⁺,Zn²⁺was respectively decreased by 28.43%,52.11% and 49.97%,which indicated that Cu²⁺ and Zn²⁺ had strong competition.The adsorption capacity of Pb²⁺with the conditions of Pb²⁺and Cu²⁺,Pb²⁺and Zn²⁺ as well as coexisting with Pb²⁺,Cu²⁺,Zn²⁺,compared with that of single Pb²⁺,decreased 25%,15.6% and 25.9%,respectively.The total amount of MHAP adsorbed heavy metal was 12mmol/L.?5?The changes of surface morphology,chemical composition were analyzed.The results showed that the mechanism of Cu²⁺ and Pb²⁺ adsorbed by MHAP was different.The main mechanismsto remove Cu²⁺ for MHAP were surface adsorption,microporous adsorption and surface complexation.However,the main mechanisms of MHAP adsorption Pb²⁺were surface adsorption,microporous adsorption and dissolved precipitation.There were two ways to produce precipitation,one was directly generated Pb₉?PO₄?₆,the other is formed Pb₁₀?PO₄?₆?OH?₂.Bonding energy intensity could reflect the combiningability between heavy ions and MHAP.Complexing key was stronger than van der Waals force,lower than the ordinary chemical bonds,which could explain the reason why MHAP to Cu²⁺ adsorption capacity was lower than Pb²⁺.